Cancer has been the subject of intense investigation in the past two decades. However, the underlying causes responsible for cancer metastasis are poorly understood and most types of cancer preventions are still limited. Effective ways to prevent cancer metastasis are urgently needed.
Antibody based therapy has proven to be effective for cancer treatment; however, this approach has been traditionally limited to extracellular or secreted proteins expressed by cancer cells1,2. Thus, a number of potential cancer or tumour markers and cancer antigens have been identified in the literature and antibody therapies have been developed against some of them.
For example, the well-known cancer therapy Herceptin (Trastuzumab) is a monoclonal antibody that can kill HER2-positive cancer cells. Herceptin binds to the HER2 (human epidermal growth factor receptor 2) antigen on the cancer cell. Likewise, Bevacizumab (Avastin™) is a monoclonal antibody targeted against vascular endothelial growth factor (VEGF), one of the growth factors implicated in the formation of new blood vessels. By inhibiting angiogenesis, Bevacizumab prevents tumour cells from receiving a constant supply of blood to receive the oxygen and nutrients the tumour needs to survive.
However, the applicability of antibody therapeutics for different cancers is not universal. One of the limitations that has prevented the general use of antibody therapeutics is the large size of antibody molecules and their consequent inability to cross the plasma or cell membrane. In the absence of modification, antibodies (including monoclonal antibodies) are only generally suitable for targeting cancer antigens located at the surface or exterior of host cells14″15. In the examples above, HER2 receptor is located on the cell surface and is hence accessible for antibody binding by Herceptin. Likewise, VEGF is secreted into the bloodstream and is able to be bound by Bevacizumab.
Most oncogenic proteins are intracellular proteins (such as intracellular phosphatases, intracellular kinases, transcription factors, etc), and have remained under-explored by the approach of antibody therapies. The long held view that antibodies are too large to penetrate cell membrane has hampered the technology of antibody therapy used in targeting intracellular proteins.
There is therefore an urgent need for effective ways of treating and preventing cancer metastasis. Antibodies have not hitherto been used for targeting intracellular antigens or cancer markers because of the inability of the antibodies to cross the cell membrane and the consequent inaccessibility of the antigen.
Antibody-based therapies have better specificity and thus improved efficacy over standard chemotherapy regimens. Because antibodies are viewed as too large to access intracellular (inside the cell) locations, antibody therapy has traditionally targeted extracellular (outside the cell or cell surface) proteins expressed by cancer cells. However, a large pool of oncogenic proteins is found within the cell (such as intracellular phosphatases/kinases and transcription factors) and has therefore not been pursued for antibody therapies.
We previously showed three different antibodies could respectively target three intracellular proteins: PRL-3 (phosphatase of regenerating liver 3), a cancer-associated phosphatase; EGFP (enhanced green fluorescent protein), a general reporter; and mT (polyomavirus middle T), the polyomavirus middle T oncoprotein (WO2011/065923). Yet, only PRL-3 intracellular phosphatase (an enzyme) has been linked to human cancer metastasis (see Saha et al., Science 294; 1343 (2001) and Wang et al., Cancer cell 18; 52-63 (2010)), the other two intracellular proteins (EGFP and middle T) are used to elucidate the general phenomena that antibody can target intracellular proteins. However, the use of oncoproteins as cancer vaccines is controversial, and a need exists to develop improved oncoprotein cancer vaccines that are more specific, and exhibit less cross-reaction with homologous proteins to reduce side effects, whilst achieving similar, or improved, therapeutic results.
Oncogenic mutations are very common in contributing to multiple human cancers. However, these oncogenic mutations are often detected in intracellular proteins or intracellular domains of cell surface proteins. Our recent works suggest an unconventional concept that intracellular oncoproteins can be targeted by therapeutic antibodies or peptide vaccination. In this new consideration generating antibodies against those specific mutations or vaccination using mutant peptides could specifically ablate cancer cells expressing respective mutated targets, but sparing normal tissues unharmed. However, making antibodies one by one in vitro specifically against countless point mutations discovered so far is very difficult and not practical. Therefore, vaccination using mutant peptides could be a choice.